These little spinach plants illustrate this idea. They were planted over a month ago and have a nice warm growing space inside a cold frame but grow very slowly because day-length has been decreasing. Combating this dilemma is simple but requires a bit of planning (or planting!) ahead. Greens and some other vegetables planted in late August and early September will have had ample time to reach maturity by now and will continue to "hang out" in the garden for a long time even when days are short and the nights get chilly.
Kale is a perfect example of cool hardy vegetable that will live in the garden. Just make sure you plant extra at then end of summer. Most cool hardy vegetables can survive light frosts. Further protection from cold frames and row covers can help stretch the season a bit longer. Cooler temperatures increase sugar and nutrient levels in vegetable making them even tastier.